Video noise reduction systems of a type which separate a video signal into two or more frequency bands for purposes of noise reduction are well known. FIG. 1 of U.S. Pat. No. 4,163,258 entitled NOISE REDUCTION SYSTEM which issued Jul. 31, 1979 to Ebihara et al. describes a known "plural frequency band" noise reduction system of the "simple coring" type. In the Ebihara et al. system a video input signal is split into high and low frequency bands by complementary high and low pass filters, the high frequency portion is cored and the bands are then recombined to provide a noise reduced video output signal in which the high frequency components thereof are "cored" and the low frequency components thereof are not disturbed or altered in any way.
One disadvantage of such a dual-band system, as explained by Ebihara et al., is that the high pass and low pass filters used to separate the video signal generally impart unequal phase shifts to the signals applied thereto and this results in phase distortion when the low frequency components are recombined with the cored high frequency components. Ebihara et al. also point out that the amplitude-frequency characteristics of such filters generally are not equal. Ebihara et al. conclude that, because of the different phase-shifts and the different amplitude characteristics in the recombined lower frequency and higher frequency components attributed to theses filters, the resultant video signal exhibits significant distortion which is detectable in the reproduced video signal. It will be further noted that the two-band coring arrangement provides no signal to noise ratio improvement for the low frequency component of the video signal being processed.
To overcome the problems noted above with conventional coring type noise reduction systems, Ebihara et al. propose a multi-band noise reduction system in which the video signal is first converted to number of time coincident samples, then transformed (by Hadamard transforms) into plural frequency bands with coring applied to all but the lowest frequency band, whereupon the processed signals are applied to an inverse-Hadamard transform matrix and are finally re-combined to provide a noise reduced video output signal. A disadvantage of such a system is that it is relatively complex.
Other workers in the noise reduction field have also attempted to improve upon the "simple" form of coring system noted above by using other forms of transforms. U.S. Pat. No. 4,523,230 entitled SYSTEM FOR CORING AN IMAGE-REPRESENTATIVE SIGNAL of Carlson et al. which issued Jun. 11, 1985 describes a multi-band spatial frequency coring system. In a preferred embodiment therein disclosed, spatial frequency transforms of the so-called "Burt Pyramid" type are used in combination with plural spatial frequency band coring. Briefly, an input signal to be noise reduced is first applied to a non-ringing, non-aliasing, localized trransfer, octave band spatial frequency spectrum analyzer which separates the video input signal into subspectra signals. Next, the subspectra signals are individually cored. Finally, the cored subspectra signals are applied to a synthesizer employing one or more non-ringing, non-aliasing filters for deriving an output image-representative signal from all of the subspectra signals. Such a system, as compared with simple coring as described above, is also relatively complex.
An alternative to the foregoing multi-band noise reduction systems is to employ single band processing. An elementary single band processor comprises just a low pass filter. Such a system, while having the virtue of simplicity, tends to remove signal as well as noise and results in a "soft" appearing picture lacking in detail even for cases where there is no noise at all.
A single band noise reduction system that is effective for improving the overall signal to noise ratio of video signals having little or no frame-to-frame motion is the well known "recursive" filter technique wherein signal to noise ratio is enhanced by frame to frame correlation methods. Briefly, (for the case of still images) by combining a number of frame delayed signals in an accumulator the signal power of the sum increases more quickly than the noise power. This results because the signal of still images is coherent from frame to frame whereas the noise is not coherent on a frame to frame basis. Accordingly, frame recursive filtering provides a real signal to noise ratio improvement for images having little or no motion. Several examples of motion adaptive frame recursive filters are described by Takahashi in U.S. Pat. No. 4,246,610 entitled NOISE REDUCTION SYSTEM FOR COLOR TELEVISION SIGNAL which issued Jan. 20, 1981. On the other hand, frame recursive filtering, as it has heretofore been implemented, requires a substantial amount of memory to implement the required frame delay.